[meteorite-list] Carbonated Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 09:41:06 2004
Message-ID: <200102052038.MAA19341_at_zagami.jpl.nasa.gov>

http://science.nasa.gov/headlines/y2001/ast04feb_1.htm

Carbonated Mars
NASA Science News

Here on Earth the only way to make carbonate rocks is with the aid of liquid
water. Finding such rocks on Mars might prove, once and for all, that the
barren Red Planet was once warm and wet.

February 5, 2001 -- A common substance found in ordinary classroom chalk
could hold the key to a puzzle of planetary proportions: the mysterious
whereabouts of water on Mars.

The brittle, white material in chalk --a form of carbonate-- may seem rather
ordinary, but finding carbonates on Mars would have some extraordinary
implications. The discovery would provide strong evidence that liquid water
once flowed on the Red Planet. Such carbonates might also harbor the fossils
of ancient Martian bacteria.

"If you were lucky enough to find some carbonates in the layered terrains on
Mars, scientists would get very excited about it," said Ken Nealson,
director of the Center for Life Detection at NASA's Jet Propulsion
Laboratory.

"It would be just a zinger of a finding."

Carbonate rocks on Earth are formed in two ways: through a purely chemical
process or via the action of living things. Both means require liquid water.

The chemical pathway involves carbon dioxide gases that dissolve in surface
waters. CO2 molecules combine with water to form carbonate ions, which in
turn join with calcium or magnesium to create a solid that settles onto the
sea floor. Limestone (CaCO3) is an example of such a carbonate. Geologic
changes can later expose such deposits, revealing beautiful features such as
the white cliff faces pictured above.

Because Mars's atmosphere contains mostly carbon dioxide, scientists would
expect liquid surface waters (if they ever existed on Mars) to produce
carbonate deposits in a similar fashion.

Another way carbonates are formed on Earth is by marine organisms that
produce carbonates for shells or other hard parts. When these organisms die,
the shells sink to the bottom, where they accumulate and eventually form a
carbonate deposit. Blackboard chalk is one example of this type of
carbonate, which comprises the majority of carbonates in our planet's crust.

"Not only are carbonates often a product of life, they preserve the life
that was in and around them very well," Nealson continued. "The whole notion
of looking for certain mineral types that ... tend to harbor life here on
Earth is an important part of the search strategy [for signs of life on
Mars]."

Roaming the entire surface of Mars searching for carbonate rocks would take
a very long time. Fortunately, carbonates can be detected from orbit by
looking at radiated heat.

Like all substances, carbonates emit heat as infrared (IR) radiation.
Carbonate compounds have a distinctive infrared signature when viewed
through an IR spectrometer.

NASA's Mars Global Surveyor spacecraft, which is currently orbiting Mars,
carries such an instrument --the "Thermal Emission Spectrometer" (TES)--
which is able to read the infrared "fingerprints" of rocks on the Martian
surface below. Scientists had hoped this sensor would find regions of
exposed carbonate among the Martian landscape.

So far, the TES has not discovered any carbonate deposits.

"If they're really not there, it's very discouraging," Nealson said. "But we
may not have seen them just because we haven't had the right instrument
yet."

An improved version of the TES will be on its way to Mars soon. Called the
Thermal Emission Imaging System (THEMIS), this new instrument will take more
detailed infrared images of the Martian surface than the TES, enabling
THEMIS to detect smaller carbonate deposits than TES can.

THEMIS will fly on board NASA's 2001 Mars Odyssey spacecraft, which is
scheduled to launch in April.

While scientists wait for the results of THEMIS, it may be that evidence for
carbonates on Mars has already been found here on Earth.

A rock from Mars, which was apparently ejected from the Red Planet by an
asteroid impact millions of years ago, came to rest in Antarctica about
13,000 years ago, where it was found by scientists in 1984. The "Mars Rock,"
also known as the "Allen Hills meteorite," caused a stir in 1996 when
scientists announced that the rock contained signs of ancient Martian
microbial life.

That conclusion has since been criticized by other scientists, but one of
the pieces of evidence cited were small patches of carbonate mineral inside
the rock. The location of the carbonate patches along with other clues
suggested that the carbonate was there millions of years ago when the rock
was still on Mars.

"We know there are carbonates (on Mars), because we see them as weathering
products in a variety of Martian meteorites," said Everett Gibson, an
astrobiologist at NASA's Johnson Space Center in Houston, Texas.

"The big question is, Where are the carbonates on the surface of Mars?
Shouldn't they be seen by some of the spectrometers that are looking at Mars
now?"

Tiny patches of carbonate like those found in the "Mars rock" would not be
detected by the thermal emission spectrometer currently in orbit around
Mars, Gibson continued. Even THEMIS's 100-meter resolution isn't likely to
reveal such diminutive deposits.

But if lakes or oceans once adorned the Martian landscape, scientists expect
that sooner or later their instruments will reveal carbonate deposits. Such
a discovery would prove, once and for all, that Mars was not always the
barren desert it is today.
Received on Mon 05 Feb 2001 03:38:58 PM PST